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1. Bristles or spines joined to some degree at the base, forming a burr Bristles free or almost free to the base, fine and thread‑like |
2 13 |
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| 2. Spines connate for more than 1/3 of distance above base | 3 | |
| Spines connate to 1/3 of distance above base | 6 | |
| 3. Burr of 1 whorl of united flattened spines surrounded by many smaller finer bristles | 4 | |
| Burr of several whorls of united flattened spines surrounded by many smaller finer bristles | 5 | |
| 4. Outer bristles and inner spines ± equal in length | *C. brownii (NT WA QLD) | |
| Outer bristles ± 1/2 length of inner spines | *C. echinatus (NSW NT WA QLD) | |
| 5. Spines slender, usually more than 30 |
*C. longispinus (NSW VIC NT SA WA QLD) C. pauciflorus var. longispinus, C. tribuloides auct. |
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| Spines broader at base, less than 25 |
*C. spinifex (NSW VIC SA WA QLD) C. pauciflorus, C. tribuloides auct. |
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| 6. Spines retrorsely barbed | 7 | |
| Spines antrorsely barbed | 9 | |
| 7. Spines grooved externally |
*C. biflorus (NT WA QLD) C. barbatus |
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| Spines not grooved externally | 8 | |
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8. Burr with one spikelet 3.8-6mm long |
C. caliculatus (NSW QLD) C. australis |
| Burr with two spikelets 8-9mm long | C. robustus (QLD) | |
| 9. Spines not extending beyond burr |
* C. setigerus (NSW NT WA QLD) Pennisetum setiger |
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| Spines or a single spine extending beyond burr |
10 |
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| 10. Spikelets c. 8 mm long |
11 |
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| Spikelets c. 5 mm long | 12 | |
| 11. Burr with a single elongated bristle and a few very short basal bristles |
C. brevisetosus (NT WA QLD)
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Burr with many bristles as long as or extending beyond spikelets, one of them at least three times longer than the others |
C. elymoides (NT WA QLD) | |
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12. Bristles joined together only at base |
* C. ciliaris (NSW NT SA WA QLD) Pennisetum ciliare |
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Bristles joined together above base for 1-3 mm |
* C. pennisetiformis (NT SA WA QLD) Pennisetum pennisetiforme |
| 13. Inflorescence concealed within uppermost leaf sheath |
* C. clandestinus (NSW VIC TAS SA WA QLD) Pennisetum clandestinum |
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| Inflorescence exserted from uppermost leaf‑sheath | 14 | |
| 14. Rachis angular |
15 |
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| Rachis cylindric |
18 |
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| 15. Involucral bristles scaberulous; rachis hair |
C. basedowii (NT WA QLD) Pennisetum basedowii |
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| Involucral bristles hairy; rachis glabrous or scaberulous | 16 | |
| 16. Spikelet sessile within involucre |
* C. polystachios (NT QLD) |
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| Spikelet pedicelled within involucre | 17 | |
| 17. Involucre with 1 spikelet |
* C. pedicellatus subsp. unispiculus (NT WA QLD) Pennisetum pedicellatum subsp. unispiculatum |
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| Involucre with 2‑5 spikelets |
* C. pedicellatus subsp. pedicellatus (NT WA ) Pennisetum pedicellatum subsp. pedicellatum |
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| 18. Annual; grain exserted at maturity from gaping spiklelet |
* C. americanus (NSW NT WA QLD) Pennisetum glaucum |
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| Perennial; grain not exserted from spikelet | 19 | |
| 19. Involucres on a stalk 1-3 mm long | 20 | |
| Involucres subsessile | 22 | |
| 20. Involucre bristles glabrous |
* C. purpurascens (NSW VIC QLD) Pennisetum alopecuroides. P. compressum |
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| Involucre bristles hairy | 21 | |
| 21. Spikelets 9-10 mm long |
* C. longisetus (NSW VIC TAS SA WA QLD) Pennisetum villosum |
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| Spikelets 5-6 mm long |
* C. setaceus (NSW VIC SA WA QLD) Pennisetum setaceum |
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22. Slender plant to 60 cm long; inflorescence c. 5 cm long |
* C. thunbergii (QLD)
Pennisetum glabrum, P.thunbergii |
| Vigorous plant exceeding 1.5 m; inflorecence 8-30 cm long | 23 | |
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23. Bristles scabrous, slightly longer than spikelets except for a single longer bristle; culms not branched |
* C. macrourus (NSW VIC TAS SA WA) Pennisetum macrourum |
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| Bristles scabrous to hairy, at least 3 times the spikelet length; culms branched |
* C. purpureus (NSW NT SA WA QLD) Pennisetum purpureum |
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| Key to Cenchrus derived from De Lisle 1963, Vickery 1975 (Cenchrus s.s and Pennisetum), Webster 1987 (Cenchrus s.s and Pennisetum) . Nomenclature from Chemisquy et al 2010. |
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